1. Field of the Invention
The present invention relates generally to improvements in or relating to connecting devices for use with components such as dental implants, or implants for artificial joint reconstructions. More particularly, the present invention is concerned with a connecting device for joining an implant or implant member, which implant member includes a bore, and a superstructure element. The invention is also concerned with an implant member which is adapted to receive the connecting device, as well as with a method of arranging the connecting device in the implant member.
The connecting device or element in accordance with the present invention is particularly suitable for implants intended for roots of teeth, however, such a connecting device is also suitable for implants associated with artificial joint reconstructions.
The connecting device for implants in accordance with the present invention is particularly adapted to distribute an introduced force, or forces, from the point of force introduction, for example, the artificial tooth or a superstructure, into the implant base, for example, the jaw bone, particularly in such a manner that localized overloading or excessive stressing of the bone is substantially prevented.
In the case of many endo-osseous implants (artificial alloplastic tooth root anchored in the bone) there often arises, after brief to periods to several years of wearing of an implant, a bone reduction at the implant base, which ultimately leads to the loss of the implant or even its fracturing.
This so-called `resorption` of the bone can be attributed only in part to insufficient biocompatibility (compatibility with the tissue) of the implant member material, or to infections. A material cause of the bone reduction, however, is due to biomechanical stressing of the implant base. Thus, a localized, frequently repeated, loading or stressing of the bone, with mechanical stress peaks, leads to the bone reduction. This applies to dental root implants as well as to bone or joint implants. During the implanting of such implant members it is, accordingly, of determining importance for a lasting existence thereof in the organism, that such damaging stress peaks will be prevented by means of suitable load distributing mechanisms.
In the case of implants for the replacement of tooth roots, as in the case of other types of implants, there arise a number of load situations which, in the case of direct frictional connection between the load which is introduced and the implant base, bearing or socket, via a rigidly situated implant, lead to such localized stress peaks and, thus, to resorption, loosening and loss of the implant.
The natural tooth is supported by an elastic fiber network, the periodontium, in a manner similar to being supported in a hammock and is there joined to the surrounding bone. By means of this mechanism the tooth can be elastically deflected when subjected either to horizontal or vertical forces, whereby the bone bearing or base is predominantly subjected, via the fibers, probably largely uniformly, to tensile stressing. Thus, the physiological deflection of an upper incisor, for example, on being subjected to a horizontal force of 5 N, is approximately 20 to 80 .mu.m. On being subjected to an equal, but vertically directed force, the root of the tooth is pressed into the base by approximately 20 to 30 .mu.m. Simultaneously there occurs, by way of mechanoreceptors in the periodontium, via cortical and subcortical control mechanisms (cerebral nerves), a coordination and limiting of the force-release of the muscles employed for chewing.
In the case of an endo-osseous implant such dental sensorium is not present and, thus, there is also not present a control of the force introduction, such that this alone may lead to overloading. Furthermore, an implant is generally form-lockingly surrounded by bone growth, or possibly even force-lockingly surrounded by bone growth. In the case of a rigid connection between the implant and the replacement of a tooth, or a bridge, the entire force applied is transmitted directly, and locally limited, via the implant, into the implant base, bearing or socket. In the case of a natural tooth, in contrast, when a point-like introduction of a load occurs, there follows, initially, a deflection, or sinking-in, respectively, of the tooth and, at a significant relation of the rows of teeth and the teeth ridges with respect to one another, very rapidly a support, or dissipation, of the chewing forces by the other teeth. This prevents an overloading of individual tooth holding apparatus. In the case of form-lockingly and force-lockingly ingrown implants this deflection possibility does not exist, and stress increases arise in the bone base and, thus, bone reduction and loss of the implant.
By way of photoelastic investigations and numerical calculations it was determined that primarily the horizontal components of the forces applied at the tooth lead to considerable load or pressure peaks, particularly at the exit edge of the implant from the bone, at the so-called `corticalis` (cortical bone). This provides an explanation for the repeatedly observed conical opening and regeneration of the bone around the implant.
In the case of a vertical loading, the forces are largely introduced symmetrically via the wall of the implant and into the bone, via the bottom of the implant. Normally in such a case localized stress peaks will not occur, however, a general overloading of the entire implant bed can also occur, since the implant is rigidly connected, in contrast to a natural tooth, to the bone.
In the FIGS. 1 to 4 there are graphically presented the results of calculations using a model (finite-element calculations), which were carried out at the Fraunhofer-Institut fur Werkstoffmechanik (Fraunhofer Institute for Materials Mechanics) at Freiburg.
FIG. 1 shows a schematic representation of the finite element model (FE-model) of an implant in the lower jaw, assuming a steady connection between the implant and the bone base, or bearing or socket.
There is shown a cross section through the jaw and the implant. The bone is nonhomogeneous and is comprised of a hard corticalis-shell and the soft inner spongiosa. The modulus of elasticity (E-modulus) of the corticalis is assumed to be 20,000 N/mm.sup.2, that of the spongiosa 2,000 N/mm.sup.2, and that of the implant (Al.sub.2 O.sub.3 -ceramic) 380,000 N/mm.sup.2.
FIG. 2 shows the distribution of the primary tensions in the bone along the boundary surface of the implant, whereby the abscissa describes the path along the surface of the implant in counter-clockwise direction. As the load there is assumed a strictly horizontally directed force of 10 N, which is applied at the point A (see FIG. 1), approximately at the level of the cutting edge of a front tooth. The curve clearly indicates the stress peaks which are generated in the corticalis.
FIGS. 3 and 4 show the distribution of stress for the case where the introduction of the horizontal force is not at point A (see FIG. 1), but at point B, or C, respectively, which corresponds approximately to the center of rotation of the implant.
A comparison of FIGS. 2 to 4 clearly indicates that the stress peaks, which occur at the transitional edge of the implant into the bone, are dependent upon the location of force introduction, i.e., the torque which is introduced. For example, if the force is applied near the center of rotation, then the stresses of the bone can be nearly completely eliminated at the transitional exit edge of the bone (FIG. 4).
2. Description of the Prior Art
Proposals to reduce, or avoid, the non-physiological pressure peaks in the bone bed are known. These are primarily based on the concept--physically arguable--of reduction of the pressure peaks by means of force-absorbing, or shock-absorbing, respectively, intermediates or buffers, which exhibit an `elastic` behavior (i.e., presumably exhibit a reduced rigidity). These buffers are comprised either of an attachment to the implant, which attachment extends into the superstructure (DE-OS No. 2 704 390, DE-OS No. 2 824 214, DE-OS No. 2 413 883, DE-OS No. 2 950 219, DE-OS No. 2 830 025; DE-OS=German Offenlegungsschrift), and/or are comprised of a pin (DE-OS No. 2 824 214), or an intermediate layer within the implant part embedded in the bone (DE-OS No. 2 413 883, U.S. Pat. No. 3,934,347). These are all made of a soft plastic or synthetic material. All such designs have only a limited effect in the redistribution of the pressure or force and, thus, in the reduction of the pressure peaks. This is due thereto that there is allowed, because of a somewhat increased mobility of the superstructure, primarily in vertical direction and with a suitable arrangement, a limited distribution of the introduced force to the neighboring teeth, or further columns of the superstructure. The danger of the horizontal force component, however, is not, or only insignificantly, reduced, since the point of force introduction remains constant, or is only slightly relocated. Thus, the known devices can not help to prevent the bone reduction. This also applies in the case of the proposals described in DE-OS No. 2 015 324, DE-PS No. 923 383 (DE-PS=German Patent) and U.S. Pat. No. 3,934,347. A redistribution of the introduced horizontal components could firstly be achieved by the special arrangement comprising a rod with a projection means and the support thereof in a suitable envelope.